Currently, the electron sources of microwave amplifier tubes are thermionic. But sources functioning on the principle of field emissions (cold cathodes) are being studied for their potentially better performance (modulation of the emission at higher frequency, immediate starting of the cathode without heating delay, compactness of the tubes, etc.). The majority of these cathodes are made up of a conducting plane surface provided with relief structures, that are also conducting and onto which the electric field is concentrated. These relief structures are electron emitters when the field at their apex is sufficiently high. These cathodes are thus purely electrically controlled: in triodes, for example, which comprise three successive electrodes, the cathode, the grid and the anode, the electrical voltage applied between the cathode and the grid is modulated in order to modulate the electron current emitted by field emission from the cathode
Several limitations result from this electrical control of the emission:    1/ The cathode surface area must be reduced to dimensions significantly smaller than the wavelength of the microwave signal, so that all the points on the cathode surface emit in phase, and therefore the modulation depth of the total emitted current is not attenuated. However, the smaller the surface area, the lower the emitted current and the more the output power is limited.    2/ Furthermore, the need for the cathode to be biased with a high negative voltage with respect to the extraction grid imposes the presence of a galvanic insulation between the cathode and the outer conductor of the input coaxial guide which is grounded. This galvanic insulation can limit the performance at high power.    3/ Moreover, in microwave tube technology, the microwave is coupled with the input cavity of the tube by a connector that is difficult to miniaturize. This is a limitation on the miniaturization of the whole cavity, and consequently, a limitation on the optimum operation.    4/ Also, the bandwidth (in particular, the maximum frequency of operation of the cathode) is limited owing to the fact that the input signal is applied to an input impedance with a significant capacitive component.